1. Field of the Invention
The present invention relates to a wireless data communication system.
2. Description of Related Art
Nowadays, when connected to an internet, generally, a TCP (transmission control protocol) is used as a transport layer protocol. The TCP is provided with a retransmission function in consideration of a case where a packet may be lost on a transmission line. In the TCP, although a retransmission timer is started when the packet is transmitted to a recipient and the retransmission timer is stopped when a confirmation response was received from the recipient, the packet is re-transmitted if the confirmation response is not received and the retransmission timer is timed out.
Further, when a terminal is connected to the internet through a public network, a PPP (point-to-point protocol) is generally used as a data link layer protocol which is a lower order layer of a TCP/IP (transmission control protocol/internet protocol). In the PPP, the data link is monitored by transmitting echo request and by receiving echo reply from the recipient. Upon transmitting the echo request, a watch timer is started, and the link of the PPP is disconnected if the echo reply is not received until the watch timer is timed out.
In this way, although a protocol stack of the TCP/IP or PPP is used when connected to the internet from the terminal through the public network, when the terminal is a wireless terminal, by using a data transfer protocol as a lower order layer of the TCP/IP or PPP protocol, loss of the data packet due to an error inherent to wireless communication is avoided.
Since the TCP/IP and PPP assume that wire connection is used, vergence of the network is considered with respect to the loss of the data packet in the TCP. Whereas, in the wireless data transfer protocol, occurrence of a burst error inherent to the wireless communication and hand-over must be considered. As a representative wireless data transfer protocol, for example, there is a PIAFS (PHS internet access forum standard) data transfer protocol used in a PHS (personal handyphone system).
The PIAFS data transfer protocol will now be explained with reference to FIG. 7. FIG. 7 is a view showing a frame format in the PIAFS data transfer protocol.
In the PIAFS data transfer protocol, data is transferred with a frame format of 640 bits as shown in FIG. 7. The frame is provided with a plurality of areas 900 to 905.
An area 900 is an area in which a frame identification element (FI) is stored. The frame identification element (FI) serves to identify either one of a negotiation frame, a synchronous frame, a control frame or a data frame. The negotiation frame is a frame used in negotiation for determining whether or not the PIAFS data transfer protocol is used. The synchronous frame is used to effect synchronization of the data frame of 640 bits in the PIAFS data transfer protocol. The control frame is used in transceiver of control data such as transceiver of various parameters of the data transfer protocol. The data frame is used for transferring the data.
As the format of the data frame, there is a format shown in FIG. 7, and, in this case, a transmission frame number (FFI) is stored in an area 902 and a request frame number (FBI) is stored in an area 902. The transmitting side continues to transmit a corresponding transmission frame so long as confirmation indicated by the request frame number (FBI) is not received.
A data length (byte number) transmitted by the frame is stored in an area 903. Data transmitted by the frame is stored in an area 904. A calculation result obtained by CRC 32 of the frame in a frame check sequence is stored in an area 905. The calculation result is stored in the transmitting side.
In the PIAFS data transfer protocol, the calculation is effected by the CRC 32 of the received frame, and the calculation result is compared with the calculation result stored in the area 905, and it is judged whether there is an error in the data received from the recipient. As a result of the judgement, if both do not coincide with each other, the received frame is discarded. Hereinafter, the case where the calculation result obtained by the CRC 32 is referred to as a CRC error.
If the wireless condition is worsened or if the hand-over occurs, the CRC error is generated continuously. In the PIAFS data transfer protocol, if the error occurs continuously for 20 frames in case of 32 Kbps data transmission and for 40 frames in case of 64 Kbps data transmission, it is regarded as out-of synchronization, and a resynchronization sequence is started. If resynchronization is established within 15 seconds at the maximum, the data transfer based on the PIAFS is continued.
Next, a system which can be connected to the internet by using a wireless data terminal will be explained with reference to FIG. 8.
As shown in FIG. 8, a wireless data terminal 701 includes a wireless controller 801 for controlling the PHS, a PIAFS protocol controller 805, a PPP protocol controller 806 and a TCP/IP protocol controller 807. Although not shown, the wireless data terminal 701 an application controller for controlling applications such as an electronic mail, a web browser and the like, as a higher order of the TCP/IP protocol controller 807.
Next, a sequence in transmitting the TCP/IP data packet from the wireless data terminal 701 will be explained. FIG. 8 is a view showing a sequence effected when the TCP/IP data packet is transmitted from the wireless data terminal 701 to a server 106 through a base station 102 and an access point 104. Incidentally, the base station 102 and the access point 104 are interconnected via the public network, and the access point 104 and the server 106 are interconnected via the internet.
It is assumed that the wireless data terminal 701 and the access point 104 are presently connected to the base station 102 via the public network. In this case, the PIAFS link and PPP link are established between the PIAFS protocol controller 805 of the wireless data terminal 701 and the access point 104.
As shown in FIG. 8, the TCP/IP protocol controller 807 transmits a data packet 811 to the PPP protocol controller 806. Upon occurring such transmission, the retransmission timer is started. The PPP protocol controller 806 which received the TCP/IP data packet 811 from the TCP/IP protocol controller 807 adds PPP header to the TCP/IP data packet 811 and transmits a PPP packet 812 to the PIAFS protocol controller 805. The PIAFS protocol controller 805 divides the PPP packet into data frames 813A to 813D each having bytes smaller than 73 bytes and transmits them to the access point 104 through the public network.
In the access point 104, the divided data frames 813A to 813D are reconstructed to restore the PPP packet and removes the PPP header and transmits it to the internet as a TCP/IP data frame 814.
When the server 106 receives the TCP/IP data frame 814 including its own address, the server transmits a TCP/IP confirmation response packet 821. The TCP/IP confirmation response packet 821 reaches the wireless data terminal 701 through a route 822 opposite to a route through which the TCP/IP data packet is transmitted. When the TCP/IP protocol controller 807 of the wireless data terminal 701 receives a TCP/IP confirmation response packet 824, the retransmission timer which was started upon transmission of the data packet is stopped.
If the confirmation response packet 824 is not received before the retransmission timer is timed out, the transmitted data packet 811 is retransmitted.
As mentioned above, in case of a moving communication network such as PHS, the CRC error may occur continuously in the PIAFS data transfer protocol if the wireless condition is worsened or if the hand-over occurs. If the CRC error occurs continuously, the PIAFS data transfer protocol starts the resynchronization sequence. However, since reestablishment of synchronization in the resynchronization sequence takes 15 seconds at the maximum, the following inconvenience will occur.
Now, such inconvenience will be explained with reference to FIG. 9. FIG. 9 is a sequence view when the inconvenience occurs during the transmission of the TCP/IP data transmission in the system shown in FIG. 8.
Although the TCP/IP data packet 811 (PPP packet 812) sent from the TCP/IP protocol controller 807 to the PIAFS protocol controller 805 through the PPP protocol controller 806 is divided into the PIAFS data frames 813A to 813D which are in turn transmitted, if the resynchronization sequences 831, 832 are started during the transmission of the data frames 813A to 813D, the data frames (813C, 813D) are not transmitted until the resynchronization sequences are finished.
Accordingly, the time when the TCP/IP data packet reaches the server 106 is delayed by a time amount corresponding to the resynchronization establishing time, in comparison with the case where the resynchronization sequences 831, 832 are not started during the transmission of the data frames.
Further, also when the confirmation response packet 821 is transferred from the server 106, if the hand-over (833) occurs between the wireless data terminal 101 and the base station 102, reach of the confirmation response packet 824 to the TCP/IP protocol controller 807 will be further delayed.
During the time period in which such TCP/IP packet is being transferred, if the resynchronization sequences are started between the wireless data terminal 101 and the base station 102, since the total time period of the resynchronization sequences are added to the time period from when the TCP/IP data packet is transmitted by the TCP/IP protocol controller 807 to when the confirmation response packet is received, in comparison with the normal case, the possibility of the fact that the retransmission timer is timed out is increased. If the retransmission timer is timed out, retransmission (811R) of the TCP/IP data packet is effected.
That is to say, in case of the conventional system, during the time period in which such TCP/IP packet is being transferred, if the hand-over occurs between the wireless data terminal 101 and the base station 102, even when the wire network prepared for the TCP/IP is completely normal, since the retransmission timer is timed out, the transmission of the data packet is effected.
Further, in the PPP protocol controller 206, although echo request is transmitted and waits echo replay with respect to recipient of the PPP link in order to watch or monitor the link, even when the resynchronization sequence is started during the transferring of the echo request and echo reply, similarly, the PPP watch timer may be timed out, and, if the watch timer is timed out, the link will be interrupted meaningless.